This invention relates to axial tomography, sometimes referred to as cross-sectional X-ray. A general discussion of the subject appears in the article "Image Reconstruction from Projections" by Richard Gordon, et al appearing in the October 1975 issue of Scientific American pages 56-68.
In a typical axial tomographic system, a detector receives radiation from an X-ray source along a plurality of sets of paths with at least some of the paths of a set traversing the object of interest, and with the sets of paths overlapping each other providing a plurality of sets of radiation detector output signals. These detector signals are then utilized in a complex computer operation to produce the desired image. This type of system is described in British Pat. No. 1,283,915, in the article "Theory of Image Reconstruction in Computed Tomography" by Rodney A. Brooks, et al appearing in Radiology 117: 561-572, December 1975, and in the article "Computerized Transaxial X-ray Tomography of the Human Body" by R. S. Ledley, et al appearing in Science, Oct. 18, 1974 Vol. 186 No. 4160.
These prior art systems utilizing a computer for data handling are relatively expensive because of the computer capacity required. Also, a significant amount of time is required in performing the computations for an image.
The prior art systems require digitizing of the detector output signals with a resultant loss in resolution. In order to improve resolution, one would have to increase the X-ray dosage which is undesirable in most instances. A typical prior art computerized axial tomography system will provide a display matrix of 180 bits by 180 bits for a brain section. In comparison, the analog system of the present invention utilizing a standard T-V monitor as the display can provide 1000 bits by 1000 bits for a brain section.
In addition, the prior art systems produce images which are degraded in quality if the object being radiographed is allowed to move, even in the slightest manner, during the sequential detection (scanning) process. Such motion causes the digital image reconstruction algorithms to produce artifacts which are difficult and time consuming to correct within the computer. In comparison, the analog system of the present invention does not suffer the same degree of difficulty, since such motion-induced artifacts are not generated during the analog image reconstruction process.
Accordingly it is an object of the present invention to provide a new and improved method and apparatus for axial tomography which may use the conventional radiation scanning configurations while improving resolution and image quality without requiring increases in X-ray dosage. A particular object is to provide such a method and apparatus which operates in an analog method rather than in a digital method, and an apparatus which is less expensive than the prior art systems.
An additional object of the present invention is to provide an analog method of image reconstruction which is applicable to tomographic images generally, whether produced by gamma ray, X-ray, or ultrasonic radiation. The detected quality of radiation can be either the transmitted X-ray beam which is a measurement of attenuation along a ray, as is the case of the prior art systems, or an emitted beam which is a measurement of source strength along a collimated ray, as is the case in nuclear medicine procedures. Ultrasonic radiation can be measured either as an intensity transmitted signal or as a temporal (time of flight) signal. The integrated velocity of the ultrasonic beam through the imaged object is a measure of the material dispersion as opposed to attenuation and constitutes additional information which can be imaged.